Silica-containing rubber compositions are effective for making fuel efficiency tires. To improve tan δ of a rebound resilience test or a viscoelastic test, which is a laboratory indicator of fuel efficiency, alkoxysilane modified solution polymerization diene rubber with silica compound is effective. However, Si—OR groups contained in the modified solution polymerization diene rubber is hydrolyzed with moisture in the air, and further causes condensation reaction, and therefore problematic in that molecular weight increases during storage and the silica reactivity, which is indispensable for improving physical properties, decreases.
On the other hand, to improve the rebound resilience and the like, it is necessary to introduce a reactive functional group reacting with silicas such as alkoxysilyl group, to the molecule of the rubber at its one end. It has been considered that fuel efficiency is improved when another one end that is starting end was also modified, that is, both terminal-modified diene rubber is bonded to silica, the movement is suppressed by the bondings. However, in fact, it has also been found that when highly reactive functional group such as an alkoxysilyl group was introduced to the both ends, agglomerated silica cannot be efficiently dispersed by kneading.
Therefore, functional groups at one terminal that does not include an alkoxysilyl group is regarded as advantageous because silicas' interaction with the rubber is relatively low in kneading, and structures crosslinking with silica or other molecular structure easily during vulcanization reaction are believed to be advantageous, but there remains still many challenges in preparations of silica-containing modified solution polymerization diene rubber stable in industrial production and with good quality.
As shown in Patent Documents 1 and 2, the inventors disclosed for the first time the production method of solution polymerization diene rubber having an alkoxysilyl group by reacting, after polymerization of styrene and butadiene with alkyl lithium as a polymerization initiator, alkoxysilane compounds having large steric hindrance and hardly hydrolyzed, and started industrial production. However, it was found later that the alkoxysilane compound being lacking polar group containing N atom or the like, and modified diene rubber of this compound is somewhat low reactivity with silica.
Patent Document 3 discloses a production of modified SBR by reacting an amino alkoxysilane compound after polymerizing styrene and butadiene, alkyllithium as a polymerization initiator, and evaluation results of only carbon black compound.
Patent Document 4 discloses silica compound SBR of good storage stability, produced after the polymerization of styrene and butadiene, with alkyllithium as a polymerization initiator, by reacting in a specific proportion of amino alkoxysilane compounds similar to those in Patent Document 3.
Patent Document 5 discloses a synthesis of a coupling SBR by adding tin tetrachloride, after the polymerization of styrene and butadiene with lithium morpholide as a polymerization initiator, and results of evaluation of the physical properties of the product but of only the carbon black compounded ones.
Patent Document 6 discloses a production method of polymers, wherein the polymers are produced by reacting the amino alkoxy silane compound after a block copolymerization of styrene and butadiene, with an alkyl lithium or the like including an amino group but not added to the silica as a polymerization initiator, and further butadiene part is hydrogenated.
Patent Document 7 and Patent Document 8 disclose results of evaluation of properties of a polymer as silica formulation reacted by amino alkoxysilane compound after the polymerization of styrene and butadiene, the amino alkyl lithium being added and reacted with small amount of monomer and then used as initiator. However, the polymerization initiator has a special structure, and therefore it is difficult in industrial production to synthesize and to manufacture stably.
Patent Document 9 discloses a SBR carbon black formulation which was coupled with the halogenated tin compound after the polymerization of styrene and butadiene with alkyl lithium as a polymerization initiator, and prior to the reaction by the amino alkoxysilane compound, wherein the amount of said halogenated tin compound is half of the equivalent amount of the used alkyl lithium.
However, in recent years the demand for improvement of low fuel consumption of cars has become more and more stronger from the viewpoints of prevention of global warming and energy issues, and the like. Although silica compound tires are improved in fuel economy compared to the carbon black compound tires, suitable alkoxysilane-modified solution polymerization diene rubber composition containing silica has a problem that a Mooney viscosity (MV) is changed during storage, and the improvement request of further low fuel consumption has become stronger.    Patent Document 1: JPH06-51746(B1)    Patent Document 2: JPH07-68307(B1)    Patent Document 3: JPH06-53768(B1)    Patent Document 4: JP2013053293(A)    Patent Document 5: JPS59-38209(A)    Patent Document 6: JP 3988495(B2)    Patent Document 7: JP 4289111(B2)    Patent Document 8: JP4655706(B2)    Patent Document 9: JP 2625876(B2)